Armadillos of the genus Dasypus are the only known animal models with morphologically intact immune systems to develop human leprosy following inoculation with Mycobacterium leprae, the species most intensively studied being D. novemcinctus Linne. Evidence has accumulated which indicates that armadillos captured in different locations may differ in susceptibility. Of a group of 24 animals captured and inoculated in Louisiana, 63% developed leprosy. Incidence of infection in 16 Louisiana armadillos inoculated in England was 69%. However, of 25 D. novemcinctus captured and inoculated in Venezuela, only one animal developed a small subcutaneous nodule. In the same experiment, it was found that 34% of a closely related species, D. sabanicola, were susceptible. This and other evidence indicates that incidence of infection is still not completely known, and that different phenotypes of armadillos may vary in susceptibility. In this investigation it is proposed to study the susceptibility to leprosy, polymorphism, and immune systems of 3 or 4 groups of armadillos from Florida, Louisiana, and Venezuela, since the genomes of these groups have not comingled in many years. Immunologic studies, with emphasis on cell mediated immune (CMI) activity, would survey 8 different CMI functions, and 6 antibody parameters. Polymorphism would be measured by electrophoresis of proteins.